sockatmark.3n (2010 09)

sockatmark(3N) sockatmark(3N)

NAME

sockatmark() - determine whether a socket is at the out-of-band mark

SYNOPSIS

#include <sys/socket.h>

int sockatmark(int s);

DESCRIPTION

The sockatmark() function determines whether the socket speciﬁed by the descriptor

s is at the out-

of-band data mark.

If the protocol for the socket supports out-of-band data by marking the stream with an out-of-band mark,

the

sockatmark() function returns 1 when all data preceding the mark has been read and the out-of-

band mark is the ﬁrst element in the receive queue. The

sockatmark() function does not remove the

mark from the stream.

X/Open Sockets Only

sockatmark() is new in UNIX 03.

RETURN VALUES

Upon successful completion, the

sockatmark() function returns a value indicating whether the socket

is at an out-of-band data mark:

1 If the protocol has marked the data stream and all data preceding the mark has been read.

0 If there is no mark, or if data precedes the mark in the receive queue.

On failure,

sockatmark() returns -1, and sets errno to indicate the error.

ERRORS

sockatmark() fails, errno is set to one of the following values:

[EBADF] The

s argument is not a valid ﬁle descriptor.

[ENOTTY] The

s argument does not specify a descriptor for a socket.

APPLICATION USAGE

The use of this function between receive operations allows an application to determine which received

data precedes the out-of-band data and which follows the out-of-band data.

There is an inherent race condition in the use of this function. On an empty receive queue, the current

read of the location might be at the "mark". However, the system has no way of knowing that the next

data segment arriving from the network is carrying the mark. Then sockatmark() returns false, and

the next read operation silently consumes the mark.

Hence, this function can only be used reliably in one or both ways:

• when the application already knows that the out-of-band data has been seen by the system

• and that it knows data is waiting to be read at the socket (by using

SIGURG or select()).

AUTHOR

sockatmark() was developed by HP and IEEE.